back
to top
Mahogany
tides
Mahogony
tides, also known as red tides, have been the focus of
much study in recent years. Many people who live near
the Chesapeake Bay have heard of something called a red
tide, but may not be exactly sure what it is. Perhaps
they have been sailing along in their boat and noticed
that the water in their wake was colored reddish brown
and looked kind of muddy.
Picture:
A red tide. Photo courtesy of Peter J.S. Franks,
Scripps Institution of Oceanography. See the original
photo
.
Well,
red tides aren't always as red as the water in the picture,
and red tides aren't really tides. The discoloration is
caused by dinoflagellates, a specific kind of phytoplankton—
the microscopic plants that float in the water. With enough
food and the right temperatures, dinoflagellates can reproduce
extremely quickly. Their rapid growth is called a phytoplankton
bloom and the large number of dinoflagellates gives the
water its reddish brown color.
One
of the main causes of red tides is the large amounts of
nutrients that run off from the surrounding land and reach
the water. If, for example, heavy rains occur after crop
fertilization, much of this fertilizer may run off into
the watershed and eventually into the Bay. Combined with
warm temperatures, these excess nutrients fuel the algal
blooms.
Picture:
Smithsonian scientist collecting a water sample of a mahogany
tide.
Although
the dinoflagellates produce oxygen in the growth phase
of a bloom, the algae use up a lot of oxygen when the
algae die and are decomposed by bacteria. The decay of
the algae can cause anoxia, a situation of low oxygen
levels in the water, which has been the cause of fish
kills when there is no oxygen left for the fish to breathe.
(How do fish breathe_ Read Watershed Radio's Fish
in the Flood.)
Another
problem of these massive algal blooms is that they 'muddy'
the water so much that sunlight can no longer reach submerged
aquatic vegetation (SAV). Without sunlight, these plants
cannot grow and they start to die. When SAVs disappear,
other aquatic animals, like fish
and the blue crab,
loose an important habitat and food source.
References
and further reading
The
Smithsonian Environmental Research Center has a phytoplankton
guide
to the Rhode River and Chesapeake Bay. The site includes
pictures of a particular blooming phytoplankton, Procentrum
minimum
and of its bloom.
Woods
Hole Oceanographic Institute
(WHOI)
also has some very good information about red tides, with
some great graphics.
For
more information about red tides, refer to the Public
Health Fact Sheet.
And
last, but not least, here are some red
tide
links.
